The Digital Fact Book - Quantel

Sub-pixelA spatial resolution smaller than that described by a pixel. Although digital images arecomposed of a matrix of pixels it can be very useful to resolve image detail to smaller thanpixel size, i.e. sub-pixel. For example, the data for generating a smooth curve on the screenneeds to be created to a finer accuracy than the pixel grid itself – otherwise the curve willlook jagged. Again, when tracking an object in a scene, executing a DVE move, or calculatinghow a macroblock in MPEG-4 AVC coding moves from one picture to another, the sizeand position of the manipulated picture or element must be calculated, and the pictureresolved, to a far finer accuracy than the that of whole pixels – otherwise the move willappear jerky or wrong.Moving by a whole lineMoving by half a lineLines of input pictureLines of output pictureSMoving an image with sub-pixel accuracy requires picture interpolation as its detail,that was originally placed on lines and pixels, now has to appear to be where none mayhave existed, e.g. between lines. The original picture has to be effectively rendered ontoan intermediate pixel/line position. The example of moving a picture down a whole lineis achieved relatively easily by re-addressing the lines of the output. But to move it byhalf a line requires both an address change and interpolation to take information fromthe adjacent lines and calculate new pixel values. Good DVEs and standards converterswork to a grid many times finer than the line/pixel structure.See also: Pixel, Tracking164